This invention relates to a safety apparatus and system to prevent B.L.E.V.E. (boiling liquid expanding vapour explosion) accidents in closed vessels containing fuels, such as propane or butane, or other liquifiable gases having a boiling point below the ambient temperature at atmospheric pressure, or other pressurized fluids. The vessels may be in a fixed installation, or may be mobile, such as a road or railway tanker.
Tragic accidents have occurred when such vessels have been engulfed in a fire. Typically, when a propane tank is in a fire, the temperature of the tank and fuel will rise, causing an increase in the pressure inside the tank, which will cause the safety relief valve to open, allowing some fuel to escape. Providing gaseous fuel is released (from the space above the surface of the liquid fuel), boiling will occur at the surface of the liquid due to the reduced pressure, thus lowering the temperature, and extracting heat from the tank into the liquid fuel. The pressure relief valve will close again when the tank pressure drops to below the design blow-off pressure (typically 250-375 p.s.i.). If sufficient heat is still applied to the tank, the relieving cycle will repeat.
Unfortunately, as the liquid level drops, there may be too long a heat conductance passage from the top of the tank to the area adjacent the liquid level that is at a safe, relatively cold temperature, that causes the top of the tank to reach a dangerously high temperature that weakens the tank material to the extent that the internal tank pressure causes the tank to rupture in a violent manner.
If liquid is discharged through the safety relief valve, a much higher amount of fuel (approximately 270 times) would be emitted, and a much smaller degree of internal cooling would be achieved, thus causing a much faster temperature rise of the tank.